Flow rates of fuel to a combustion appliance can affect the efficiency and emissions of the combustion appliance. Examples of combustion appliances can include furnaces, water heaters, boilers, direct/in-direct make-up air heaters, power/jet burners and any other residential, commercial or industrial combustion appliance. In many cases, a combustion appliance can be modulated over a plurality of burner loads, with each burner load requiring a different flow rate of fuel resulting in a different heat output. At higher burner loads, more fuel and more air are typically provided to the burner, and at lower burner loads less fuel and less air are typically provided to the burner.
It can be a challenge to provide a desired gas flow to the burner when the inlet gas pressure of the gas source changes over time. To address this, some systems provide a low gas pressure switch and a high gas pressure switch. When the gas pressure of the gas source falls below a low gas pressure threshold, the low gas pressure switch switches and closes the gas valve, thereby shutting down the burner of the combustion appliance. Likewise, when the gas pressure of the gas source rises above a high gas pressure threshold, the high gas pressure switch switches and closes the gas valve, thereby shutting down the burner of the combustion appliance. Shutting down the burner of the combustion appliance because of pressure changes in the gas pressure of the gas source is highly undesirable. What would be desirable are improved systems and methods for operating a combustion appliance even when the inlet gas pressure of the gas source changes substantially over time.